Abstract

Experimental results are presented of ionisation ( \alpha ) and electron attachment (( \eta ) coefficients evaluated from the steady-state Townsend current growth curves for $SF_6N_2$ and $CCl_2F_2-N_2$ mixtures over the range 60<or=E/P<or=240 (where E is the electric field in V $cm^{-1}$ and P is the pressure in Torr reduced to 20 degrees C). In both the mixtures the attachment coefficients $(\eta _ {mix})$ evaluated were found to follow the relationship; $\eta _{mix}$= \eta (1-exp(-\beta F/(100-F))) where \eta is the attachment coefficient of pure electronegative gas, F is the fraction of the electronegative gas in the mixture and beta is a constant. The ionisation coefficients $( \alpha _{mix})$ generally obeyed the relationship $\alpha _{mix}$= $\alpha _{N2}(1-F/100)$ + $\alpha _A(F/100)$ where $\alpha _{N2}$ and $\alpha _A$ are the ionisation coefficients of nitrogen and the attaching gas respectively. However, in case of $CCl_2F_2-N_2$ mixtures, there were maxima in the $\alpha _{mix}$ values for $CCl_2F_2$ concentrations varying between 10% and 30% at all values of E/P investigated. Effective ionisation coefficients ( \alpha - \eta )/P obtained in these binary mixtures show that the critical E/P (corresponding to ( \alpha - \eta )/P=0) increases with increase in the concentration of the electronegative gas up to 40%. Further increase in the electronegative gas content does not seem to alter the critical E/P.